Nitrogen oxides, carbon monoxide, and hydrocarbons are toxic and environmentally damaging byproducts found in the exhaust gas from internal combustion engines. Methods of catalytically converting nitrogen oxides, carbon monoxide, and hydrocarbons into less harmful compounds include the simultaneous conversion of these byproducts (i.e., “three-way conversion” or “TWC”). Specifically, nitrogen oxides are converted to nitrogen and oxygen, carbon monoxide is converted to carbon dioxide, and hydrocarbons are converted to carbon dioxide and water.
It has generally been found that TWC increases catalytic activity and efficiency and, thus, aids in meeting emission standards for automobiles and other vehicles. In order to achieve an efficient three-way conversion of the toxic components in the exhaust gas, conventional TWC catalysts contain large quantities of precious metals, such as Pd, Pt and Rh, dispersed on suitable oxide carriers. Typically, conventional TWC catalysts use precious metal catalysts at concentrations in the range of 30-300 g/ft3, with Rh, being used in the range of 5-30 g/ft3.
Commonly used catalyst systems suffer from several drawbacks. For example, commonly used TWC catalyst systems require precious metal catalysts in order to efficiently carry out the TWC. Such precious metals are expensive, can be inefficient, and have been shown to degrade over time/use.
There have been several previous attempts at improving the light-off performance of catalyst systems. Such attempts have tried to address problems relating to inefficiency of precious metal catalysts at lower temperatures and the degradation of such catalysts as a result of exposure to high temperatures. For example, some approaches utilize higher loadings of active precious metal catalysts (e.g., Rh) with predictable increases in cost. Other approaches have utilized substrate structures with a higher channel density (and, thus, higher amounts of precious metal catalyst). These approaches not only suffer from increases in cost, but also from higher back pressure. The higher back pressure, which is an artifact of the fact that the higher channel density decreases the amount of space through which exhaust may pass, results in an increase in fuel usage. A third approach has been to use a dual TWC system. Such TWC systems comprise a first TWC catalyst placed near the engine (i.e., a close coupled “CC” catalyst), thus exposing it to the engine's heat exhaust and allowing it to reach light-off temperature more quickly and a second, larger, TWC catalyst placed further away from the engine (e.g., under the floor of the vehicle) where there additional space allows for the placement of larger TWC catalysts systems. While such techniques lead to improved TWC catalyst efficiencies, they tend to decrease the lifespan of at least the CC TWC catalyst by exposing it to higher temperatures. In addition, CC TWC catalysts suffer from increased poisoning of the precious metal catalysts by virtue of their increased exposure to sulfur or phosphorous in engine exhaust. Thus, there is a trade-off between increasing catalyst efficiency at the expense of decreasing lifespan and, thus, requiring the expensive replacement of TWC catalysts.
Other methods for improving light-off performance focused on modifying the layout of the PGM catalysts in CC TWC catalysts. For example, some methods place additional or extra PGM catalysts at the front of the CC TWC catalysts as a further means of quickly bringing catalysts to their light-off temperatures. As can be expected, such catalyst designs suffer from the same drawbacks discussed above decreased lifespan by thermal degradation of the catalyst and poisoning of the catalysts by virtue of the fact that they are exposed to higher amounts of upstream exhaust—in addition to the fact that they require increased amounts of expensive PGM catalysts.
Thus, there is a need for catalyst formulations which have increased conversion efficiencies without requiring additional amounts of precious metals.